Metro
It was mainly the lack two-drops and overwhelming amount of bad one-drops that concerned me. The overall card quality was slightly below par, so I expected to fall behind in the early game and then never catch up.
Believe me when I say I've had better decks fair a lot worse.
On mage: it's widely considered to be the best class in arena, so when I say it's overrated I mean that it's not at the top. I think that the reputation mage has is due to noobs not playing around flamestrike, getting owned by flamestrike, and then crying about flamestrike being OP. It's certainly a strong class if you can grab it's good spells, but the same can be said about any class.
On drafting: a big determiner for arena is how well the draft goes for you. An obvious but overlooked factor in drafting is the ratio of good cards to bad cards in a classes card pool. On the outset, looking at shaman you'd think it would be a very good class for arena. Good hero power for arena. A strong weapon. Lots of great cards for keeping up in tempo (flame elly, lightning, hex). But there's a major downside: there's a disproportionate amount of trash cards in the card pool. Lots of windfury shit. Wierd cards like totemic might and ancestral healing. The class is great when you can get it's good cards, but there's a high likelihood that you'll end up being weighed down by sub-par picks due to the draft pool.
Mages are perhaps on the low end of things when it comes to card pool. They have some good stuff, but at the same time they have five secrets, all of which are bad, two minions that synergize with secrets, and some other typical junk that all classes have. I'm not about to tally up the ratios for all classes to see if mage is on the lower end, but my intuition tells me that it is so.
Of course there are lots of other factors as well -- far too many for analysis -- I just think it's food for thought to consider these ratios.
My own runs as mage have been mediocre, and according to Trump mage isn't his best arena class either. Among skilled players, it seems that paladin is consistently near the top, and that priest is up there as well. Incidentally, those two classes seem to have less trashy cards than mage.